High density integrated circuits have evolved in recent years including increasing transistor density and clock speed. The result of this trend is an increase in the power density of modern microprocessors, and an emerging need for new cooling technologies. At Stanford, research into 2-phase liquid cooling began in 1998, with a demonstration of closed-loop systems capable of 130 W heat removal. One key element of this system is an electrokinetic pump, which was capable of fluid flow on the order of ten of ml/min against a pressure head of more than one atmosphere with an operating voltage of 100V.
This demonstration was all carried out with liquid-vapor mixtures in the microchannel heat exchangers because there was insufficient liquid flow to capture all the generated heat without boiling. Conversion of some fraction of the liquid to vapor imposes a need for high-pressure operation, and increases the operational pressure requirements for the pump. Furthermore, two phase flow is less stable during the operation of a cooling device and can lead to transient fluctuations and difficulties in controlling the chip temperature. The pump in that demonstration was based on porous glass filters that are several mm thick. A disadvantage of these structures is that the pore density, structure, and mean diameter is not uniform and also not easily reproduced in a low-cost manufacturing process. Furthermore, the fluid path in these structures is highly tortuous, leading to lower flow rates for a given thickness of pump. Porous ceramic structures with nominally the same character were shown to exhibit pumping characteristics which varied by large amounts.
What is needed is an electrokinetic pumping element that would provides a relatively large flow and pressure within a compact structure and offer much better uniformity in pumping characteristics.